1. Field of the Invention
the present invention relates to a semiconductor wafer with uneven distribution of crystal lattice defects, and to a process for producing this wafer.
2. The Prior Art
Silicon crystals, in particular for the production of semiconductor wafers, are preferably obtained by pulling a seed crystal from a silicon melt, which is generally provided inside a quartz glass crucible. This so-called Czochralski crucible-pulling pulling process is described in Detail in, for example, W. Zulehner and D. Huber, Czochralski-Grown Silicon, Crystals 8, Springer Verlag Berlin-Heidelberg 1982.
Due to the reaction of the quartz glass crucible with the molten silicon during the crucible-pulling process, oxygen is included as the dominant impurity in the growing silicon crystal. concentration of oxygen is usually so high that, after the crystal has cooled, it is insupersaturated form. In subsequent treatments, the oxygen is deposited in the form of oxygen cipitates. These precipitations have both advantages and disadvantages. the so-called gettering properties of the oxygen precipitates are an advantage.
This is understood to mean that, for example, metallic impurities in the semiconductor wafer are bonded to the oxygen precipitates. These impurities are thus removed from the layer which is close to the surface and is relevant for compenents. A drawback is the oxygen precipitates in the layer which is close to the surface and is relevant for componenets interfere with the function of the components which are manufactured on the semiconductor wafer. Consequentlly, it is desirable for a precipitate-free zone, PFZ, also known as a denuded zone, DZ, to be formed in the vicinity of the surface. It is also desirable for a high concentration of precipitates to be formed in the interior of the semiconductor wafer, known as the bulk.
The prior art, for example in xe2x80x9cOygen in Siliconxe2x80x9d F. Shimura, Semiconductors and Semimatarials Vol. 42, Academic Press, San Diego, 1994, has disclosed how the outdiffusionof the oxygen near the surface is achieved in a heat treatment at temperatures of preferably over 1100xc2x0 C. As a result of this outdiffusion, the concentration of oxygen is in the layer close to the surface falls so far that there is no longer any precipitation, and consequently a PFZ is generated. This heat treatment was in most cases directly intergrated in the porcesses for producing components. In modern processes, however, these high temperatures are no longer used, and consequently the required outdiffusion is brout about by an additional heat-treatment treatment step.
The oxygen precipitation takes place substantially in two steps:
(1) formation of nucleation centers for oxygen precipitates, so-called nuclei; and
(2) growth of these centers to form detectable oxygen precipitates.
During subsequent heat treatments, the size of these nuclei can be modified in such a way that those which have a larger radius than the so-called xe2x80x9ccritical radiusxe2x80x9d grow into oxygen precipitates. On the other hand, nuclei with a smaller tadius break down (are dissolved). The growth of nuclei with a radius greater than rc takes place at elevated temperature and is substantially limited by the diffusionof oxygen. A generally accepted model (cr., for example, Vanhellemont et al., J. Appl. Phys. 62, p. 3960, 1987) describes the critical radius rc as a function of the temperature, the oxygen supersaturation and the concentration of vacancies. concentration is understood to mean particles per unit volume. A high oxygen concentrationand/or a high vacancy supersaturation simplifies or accelerates the precipitation of oxygen and leads to a higher concentration of precipates. Furthermore, the concentration or size of the precipates, in particular in semiconductor wafers, depends on heating and cooling rates during thermalfurnace processes, in particular during the so-called RTA (rapid thermal annealing) processes or an epitaxial deposition of a thin crystalline film on the wafer. During these heat treatments, semiconductor wafers are heated to temperatures of up to 1300xc2x0 C. within a few seconds and are then cooled at rates of up to 300xc2x0 C./sec.
The oxygen concentration, the vacancy concentration, the interstitial concentration, the dopant concentration and the concentration of existing precipitation nuclei, such as for example carbon atoms, also influence the precipitation of oxygen.
WO 98/38675 discloses a semiconductor wafer with an uneven distribution of crystal lattice vacancies, which is obtained by means of heat treatment. The maximum level of this vacancy profile generated in this way is situated in the bulk of the semiconductor wafer and the profile decreases considerably toward the surfaces. During subsequent heat-treatment processes, in particular 800xc2x0 C. for 3 h and 1000xc2x0 C. for 16 h, the oxygen precipitation follows this profile. The result is a PFZ without prior outdiffusion of the oxygen and oxygen precipitates in the bulk of the semiconductor wafer. According to WO 98/38675, the concentration of oxygen precipitates is set by means of the concentration of vacancies, and the depth of the precipitates is set by means of the cooling rate following the heat treatment. A drawback of this semiconductor wafer is that vacancies in the crystal lattice have proven highly mobile, in particular during a heat treatment at temperatures  greater than 1000xc2x0 C. Consequently, the profile of the vacancies becomes xe2x80x9cblurredxe2x80x9d, and consequently so does the profile of the oxygen precipitates during subsequent heat treatments.
If oxygen-containing semiconductor wafers, which have neither been subjected to a high-temperature process of  greater than 1050xc2x0 C. for outdiffusion of the oxygen nor to a heat treatment as described in WO 98/38675, are subjected to a heat treatment in a temperature range of preferably between 450xc2x0 C. and 850xc2x0 C. for preferably a few hours, subsequent heat treatments, for example at 780xc2x0 C. for 3 h and 1000xc2x0 C. for 16 h, do not produce a PFZ. Instead of producing a PFZ rather only a homogeneous precipitate density is produced in the silicon wafer. This means that a homogeneous concentration of the nucleation centers for the subsequent oxygen concentration is all that has been achieved by the treatment in the range of preferably between 450xc2x0 C. and 850xc2x0 C.
Therefore, it is an object of the present invention to provide a semiconductor wafer, and a process for producing this wafer, which has a stable profile of nucleation centers for oxygen precipitates and can thus be used as a basis for a semiconductor wafer with an improved internal gettering action.
This object is achieved according to the invention by means of a semiconductor wafer having a frontside 1, a back side 2, a top layer 3, a bottom layer 4, an upper inner layer 5 lying beneath the top layer 3, a lower inner layer 6 lying above the bottom layer 4, a central region 7 between the layers 5 and 6, and an uneven distribution of crystal lattice defects, wherein the crystal lattice defects are substitutionally or interstitially included nitrogen or vacancies.
The concentration of nitrogen preferably exhibits a maximum (max1) in the central region 7 and preferably decreases uniformly toward the front side 1 and toward the rear side 2. At its maximum (max1), the concentration of the nitrogen is preferably between 1011 and 1015 atoms/cm1.